<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>

<body>
<input type="text" id="i1">
<input type="text" id="i2">
<input type="button" value="加" onclick="f(1)">
<input type="button" value="减" onclick="f(2)">
<input type="button" value="乘" onclick="f(3)">
<input type="button" value="除" onclick="f(4)">
<div>
  结果:<span></span>
</div>

<script>
  function f(x) {
    let i1 = document.querySelector("#i1");
    let i2 = document.querySelector("#i2");
    let s = document.querySelector("span");
    //记得判断是否为数字
    if(isNaN(i1.value) || isNaN(i2.value)){
      s.innerText="输入有误!"
      //记得返回,不然白玩了,就给覆盖了
      //return只能结束当前方法,不能结束多层
      return;
    }
    //选择分支
    switch (x){
      case 1:
        s.innerText = i1.value*1+i2.value*1;
        break;
      case 2:
        s.innerText = i1.value - i2.value;
        break;
      case 3:
        s.innerText = i1.value*i2.value;
        break;
      case 4:
        s.innerText = i1.value/i2.value;
    }
  }

</script>
</body>

</html>